##Author: Guangfu Wang.
##Note: this module is from https://ceres-solver.googlesource.com/ceres-solver/+/1.12.0/cmake/FindGflags.cmake

# Ceres Solver - A fast non-linear least squares minimizer
# Copyright 2015 Google Inc. All rights reserved.
# http://ceres-solver.org/
#
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions are met:
#
# * Redistributions of source code must retain the above copyright notice,
#   this list of conditions and the following disclaimer.
# * Redistributions in binary form must reproduce the above copyright notice,
#   this list of conditions and the following disclaimer in the documentation
#   and/or other materials provided with the distribution.
# * Neither the name of Google Inc. nor the names of its contributors may be
#   used to endorse or promote products derived from this software without
#   specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
# ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
# LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
# ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
# POSSIBILITY OF SUCH DAMAGE.
#
# Author: alexs.mac@gmail.com (Alex Stewart)
#
# FindGflags.cmake - Find Google gflags logging library.
#
# This module will attempt to find gflags, either via an exported CMake
# configuration (generated by gflags >= 2.1 which are built with CMake), or
# by performing a standard search for all gflags components.  The order of
# precedence for these two methods of finding gflags is controlled by:
# GFLAGS_PREFER_EXPORTED_GFLAGS_CMAKE_CONFIGURATION.
#
# This module defines the following variables:
#
# GFLAGS_FOUND: TRUE iff gflags is found.
# GFLAGS_INCLUDE_DIRS: Include directories for gflags.
# GFLAGS_LIBRARIES: Libraries required to link gflags.
# GFLAGS_NAMESPACE: The namespace in which gflags is defined.  In versions of
#                   gflags < 2.1, this was google, for versions >= 2.1 it is
#                   by default gflags, although can be configured when building
#                   gflags to be something else (i.e. google for legacy
#                   compatibility).
# FOUND_INSTALLED_GFLAGS_CMAKE_CONFIGURATION: True iff the version of gflags
#                                             found was built & installed /
#                                             exported as a CMake package.
#
# The following variables control the behaviour of this module when an exported
# gflags CMake configuration is not found.
#
# GFLAGS_PREFER_EXPORTED_GFLAGS_CMAKE_CONFIGURATION: TRUE/FALSE, iff TRUE then
#                           then prefer using an exported CMake configuration
#                           generated by gflags >= 2.1 over searching for the
#                           gflags components manually.  Otherwise (FALSE)
#                           ignore any exported gflags CMake configurations and
#                           always perform a manual search for the components.
#                           Default: TRUE iff user does not define this variable
#                           before we are called, and does NOT specify either
#                           GFLAGS_INCLUDE_DIR_HINTS or GFLAGS_LIBRARY_DIR_HINTS
#                           otherwise FALSE.
# GFLAGS_INCLUDE_DIR_HINTS: List of additional directories in which to
#                           search for gflags includes, e.g: /timbuktu/include.
# GFLAGS_LIBRARY_DIR_HINTS: List of additional directories in which to
#                           search for gflags libraries, e.g: /timbuktu/lib.
#
# The following variables are also defined by this module, but in line with
# CMake recommended FindPackage() module style should NOT be referenced directly
# by callers (use the plural variables detailed above instead).  These variables
# do however affect the behaviour of the module via FIND_[PATH/LIBRARY]() which
# are NOT re-called (i.e. search for library is not repeated) if these variables
# are set with valid values _in the CMake cache_. This means that if these
# variables are set directly in the cache, either by the user in the CMake GUI,
# or by the user passing -DVAR=VALUE directives to CMake when called (which
# explicitly defines a cache variable), then they will be used verbatim,
# bypassing the HINTS variables and other hard-coded search locations.
#
# GFLAGS_INCLUDE_DIR: Include directory for gflags, not including the
#                     include directory of any dependencies.
# GFLAGS_LIBRARY: gflags library, not including the libraries of any
#                 dependencies.
# Reset CALLERS_CMAKE_FIND_LIBRARY_PREFIXES to its value when FindGflags was
# invoked, necessary for MSVC.
macro(GFLAGS_RESET_FIND_LIBRARY_PREFIX)
    if (MSVC AND CALLERS_CMAKE_FIND_LIBRARY_PREFIXES)
        set(CMAKE_FIND_LIBRARY_PREFIXES "${CALLERS_CMAKE_FIND_LIBRARY_PREFIXES}")
    endif()
endmacro(GFLAGS_RESET_FIND_LIBRARY_PREFIX)
# Called if we failed to find gflags or any of it's required dependencies,
# unsets all public (designed to be used externally) variables and reports
# error message at priority depending upon [REQUIRED/QUIET/<NONE>] argument.
macro(GFLAGS_REPORT_NOT_FOUND REASON_MSG)
    unset(GFLAGS_FOUND)
    unset(GFLAGS_INCLUDE_DIRS)
    unset(GFLAGS_LIBRARIES)
    # Do not use unset, as we want to keep GFLAGS_NAMESPACE in the cache,
    # but simply clear its value.
    set(GFLAGS_NAMESPACE "" CACHE STRING
            "gflags namespace (google or gflags)" FORCE)
    # Make results of search visible in the CMake GUI if gflags has not
    # been found so that user does not have to toggle to advanced view.
    mark_as_advanced(CLEAR GFLAGS_INCLUDE_DIR
            GFLAGS_LIBRARY
            GFLAGS_NAMESPACE)
    gflags_reset_find_library_prefix()
    # Note <package>_FIND_[REQUIRED/QUIETLY] variables defined by FindPackage()
    # use the camelcase library name, not uppercase.
    if (Gflags_FIND_QUIETLY)
        message(STATUS "Failed to find gflags - " ${REASON_MSG} ${ARGN})
    elseif (Gflags_FIND_REQUIRED)
        message(FATAL_ERROR "Failed to find gflags - " ${REASON_MSG} ${ARGN})
    else()
        # Neither QUIETLY nor REQUIRED, use no priority which emits a message
        # but continues configuration and allows generation.
        message("-- Failed to find gflags - " ${REASON_MSG} ${ARGN})
    endif ()
    return()
endmacro(GFLAGS_REPORT_NOT_FOUND)
# Verify that all variable names passed as arguments are defined (can be empty
# but must be defined) or raise a fatal error.
macro(GFLAGS_CHECK_VARS_DEFINED)
    foreach(CHECK_VAR ${ARGN})
        if (NOT DEFINED ${CHECK_VAR})
            message(FATAL_ERROR "Ceres Bug: ${CHECK_VAR} is not defined.")
        endif()
    endforeach()
endmacro(GFLAGS_CHECK_VARS_DEFINED)
# Use check_cxx_source_compiles() to compile trivial test programs to determine
# the gflags namespace.  This works on all OSs except Windows.  If using Visual
# Studio, it fails because msbuild forces check_cxx_source_compiles() to use
# CMAKE_BUILD_TYPE=Debug for the test project, which usually breaks detection
# because MSVC requires that the test project use the same build type as gflags,
# which would normally be built in Release.
#
# Defines: GFLAGS_NAMESPACE in the caller's scope with the detected namespace,
#          which is blank (empty string, will test FALSE is CMake conditionals)
#          if detection failed.
function(GFLAGS_CHECK_GFLAGS_NAMESPACE_USING_TRY_COMPILE)
    # Verify that all required variables are defined.
    gflags_check_vars_defined(
            GFLAGS_INCLUDE_DIR GFLAGS_LIBRARY)
    # Ensure that GFLAGS_NAMESPACE is always unset on completion unless
    # we explicitly set if after having the correct namespace.
    set(GFLAGS_NAMESPACE "" PARENT_SCOPE)
    include(CheckCXXSourceCompiles)
    # Setup include path & link library for gflags for CHECK_CXX_SOURCE_COMPILES.
    set(CMAKE_REQUIRED_INCLUDES ${GFLAGS_INCLUDE_DIR})
    set(CMAKE_REQUIRED_LIBRARIES ${GFLAGS_LIBRARY} ${GFLAGS_LINK_LIBRARIES})
    # First try the (older) google namespace.  Note that the output variable
    # MUST be unique to the build type as otherwise the test is not repeated as
    # it is assumed to have already been performed.
    check_cxx_source_compiles(
            "#include <gflags/gflags.h>
     int main(int argc, char * argv[]) {
       google::ParseCommandLineFlags(&argc, &argv, true);
       return 0;
     }"
            GFLAGS_IN_GOOGLE_NAMESPACE)
    if (GFLAGS_IN_GOOGLE_NAMESPACE)
        set(GFLAGS_NAMESPACE google PARENT_SCOPE)
        return()
    endif()
    # Try (newer) gflags namespace instead.  Note that the output variable
    # MUST be unique to the build type as otherwise the test is not repeated as
    # it is assumed to have already been performed.
    set(CMAKE_REQUIRED_INCLUDES ${GFLAGS_INCLUDE_DIR})
    set(CMAKE_REQUIRED_LIBRARIES ${GFLAGS_LIBRARY} ${GFLAGS_LINK_LIBRARIES})
    check_cxx_source_compiles(
            "#include <gflags/gflags.h>
     int main(int argc, char * argv[]) {
        gflags::ParseCommandLineFlags(&argc, &argv, true);
        return 0;
     }"
            GFLAGS_IN_GFLAGS_NAMESPACE)
    if (GFLAGS_IN_GFLAGS_NAMESPACE)
        set(GFLAGS_NAMESPACE gflags PARENT_SCOPE)
        return()
    endif (GFLAGS_IN_GFLAGS_NAMESPACE)
endfunction(GFLAGS_CHECK_GFLAGS_NAMESPACE_USING_TRY_COMPILE)
# Use regex on the gflags headers to attempt to determine the gflags namespace.
# Checks both gflags.h (contained namespace on versions < 2.1.2) and
# gflags_declare.h, which contains the namespace on versions >= 2.1.2.
# In general, this method should only be used when
# GFLAGS_CHECK_GFLAGS_NAMESPACE_USING_TRY_COMPILE() cannot be used, or has
# failed.
#
# Defines: GFLAGS_NAMESPACE in the caller's scope with the detected namespace,
#          which is blank (empty string, will test FALSE is CMake conditionals)
#          if detection failed.
function(GFLAGS_CHECK_GFLAGS_NAMESPACE_USING_REGEX)
    # Verify that all required variables are defined.
    gflags_check_vars_defined(GFLAGS_INCLUDE_DIR)
    # Ensure that GFLAGS_NAMESPACE is always undefined on completion unless
    # we explicitly set if after having the correct namespace.
    set(GFLAGS_NAMESPACE "" PARENT_SCOPE)
    # Scan gflags.h to identify what namespace gflags was built with.  On
    # versions of gflags < 2.1.2, gflags.h was configured with the namespace
    # directly, on >= 2.1.2, gflags.h uses the GFLAGS_NAMESPACE #define which
    # is defined in gflags_declare.h, we try each location in turn.
    set(GFLAGS_HEADER_FILE ${GFLAGS_INCLUDE_DIR}/gflags/gflags.h)
    if (NOT EXISTS ${GFLAGS_HEADER_FILE})
        gflags_report_not_found(
                "Could not find file: ${GFLAGS_HEADER_FILE} "
                "containing namespace information in gflags install located at: "
                "${GFLAGS_INCLUDE_DIR}.")
    endif()
    file(READ ${GFLAGS_HEADER_FILE} GFLAGS_HEADER_FILE_CONTENTS)
    string(REGEX MATCH "namespace [A-Za-z]+"
            GFLAGS_NAMESPACE "${GFLAGS_HEADER_FILE_CONTENTS}")
    string(REGEX REPLACE "namespace ([A-Za-z]+)" "\\1"
            GFLAGS_NAMESPACE "${GFLAGS_NAMESPACE}")
    if (NOT GFLAGS_NAMESPACE)
        gflags_report_not_found(
                "Failed to extract gflags namespace from header file: "
                "${GFLAGS_HEADER_FILE}.")
    endif (NOT GFLAGS_NAMESPACE)
    if (GFLAGS_NAMESPACE STREQUAL "google" OR
            GFLAGS_NAMESPACE STREQUAL "gflags")
        # Found valid gflags namespace from gflags.h.
        set(GFLAGS_NAMESPACE "${GFLAGS_NAMESPACE}" PARENT_SCOPE)
        return()
    endif()
    # Failed to find gflags namespace from gflags.h, gflags is likely a new
    # version, check gflags_declare.h, which in newer versions (>= 2.1.2) contains
    # the GFLAGS_NAMESPACE #define, which is then referenced in gflags.h.
    set(GFLAGS_DECLARE_FILE ${GFLAGS_INCLUDE_DIR}/gflags/gflags_declare.h)
    if (NOT EXISTS ${GFLAGS_DECLARE_FILE})
        gflags_report_not_found(
                "Could not find file: ${GFLAGS_DECLARE_FILE} "
                "containing namespace information in gflags install located at: "
                "${GFLAGS_INCLUDE_DIR}.")
    endif()
    file(READ ${GFLAGS_DECLARE_FILE} GFLAGS_DECLARE_FILE_CONTENTS)
    string(REGEX MATCH "#define GFLAGS_NAMESPACE [A-Za-z]+"
            GFLAGS_NAMESPACE "${GFLAGS_DECLARE_FILE_CONTENTS}")
    string(REGEX REPLACE "#define GFLAGS_NAMESPACE ([A-Za-z]+)" "\\1"
            GFLAGS_NAMESPACE "${GFLAGS_NAMESPACE}")
    if (NOT GFLAGS_NAMESPACE)
        gflags_report_not_found(
                "Failed to extract gflags namespace from declare file: "
                "${GFLAGS_DECLARE_FILE}.")
    endif (NOT GFLAGS_NAMESPACE)
    if (GFLAGS_NAMESPACE STREQUAL "google" OR
            GFLAGS_NAMESPACE STREQUAL "gflags")
        # Found valid gflags namespace from gflags.h.
        set(GFLAGS_NAMESPACE "${GFLAGS_NAMESPACE}" PARENT_SCOPE)
        return()
    endif()
endfunction(GFLAGS_CHECK_GFLAGS_NAMESPACE_USING_REGEX)
# Protect against any alternative find_package scripts for this library having
# been called previously (in a client project) which set GFLAGS_FOUND, but not
# the other variables we require / set here which could cause the search logic
# here to fail.
unset(GFLAGS_FOUND)
# -----------------------------------------------------------------
# By default, if the user has expressed no preference for using an exported
# gflags CMake configuration over performing a search for the installed
# components, and has not specified any hints for the search locations, then
# prefer a gflags exported configuration if available.
if (NOT DEFINED GFLAGS_PREFER_EXPORTED_GFLAGS_CMAKE_CONFIGURATION
        AND NOT GFLAGS_INCLUDE_DIR_HINTS
        AND NOT GFLAGS_LIBRARY_DIR_HINTS)
    message(STATUS "No preference for use of exported gflags CMake configuration "
            "set, and no hints for include/library directories provided. "
            "Defaulting to preferring an installed/exported gflags CMake configuration "
            "if available.")
    set(GFLAGS_PREFER_EXPORTED_GFLAGS_CMAKE_CONFIGURATION TRUE)
endif()
if (GFLAGS_PREFER_EXPORTED_GFLAGS_CMAKE_CONFIGURATION)
    # Try to find an exported CMake configuration for gflags, as generated by
    # gflags versions >= 2.1.
    #
    # We search twice, s/t we can invert the ordering of precedence used by
    # find_package() for exported package build directories, and installed
    # packages (found via CMAKE_SYSTEM_PREFIX_PATH), listed as items 6) and 7)
    # respectively in [1].
    #
    # By default, exported build directories are (in theory) detected first, and
    # this is usually the case on Windows.  However, on OS X & Linux, the install
    # path (/usr/local) is typically present in the PATH environment variable
    # which is checked in item 4) in [1] (i.e. before both of the above, unless
    # NO_SYSTEM_ENVIRONMENT_PATH is passed).  As such on those OSs installed
    # packages are usually detected in preference to exported package build
    # directories.
    #
    # To ensure a more consistent response across all OSs, and as users usually
    # want to prefer an installed version of a package over a locally built one
    # where both exist (esp. as the exported build directory might be removed
    # after installation), we first search with NO_CMAKE_PACKAGE_REGISTRY which
    # means any build directories exported by the user are ignored, and thus
    # installed directories are preferred.  If this fails to find the package
    # we then research again, but without NO_CMAKE_PACKAGE_REGISTRY, so any
    # exported build directories will now be detected.
    #
    # To prevent confusion on Windows, we also pass NO_CMAKE_BUILDS_PATH (which
    # is item 5) in [1]), to not preferentially use projects that were built
    # recently with the CMake GUI to ensure that we always prefer an installed
    # version if available.
    #
    # [1] http://www.cmake.org/cmake/help/v2.8.11/cmake.html#command:find_package
    find_package(gflags QUIET
            NO_MODULE
            NO_CMAKE_PACKAGE_REGISTRY
            NO_CMAKE_BUILDS_PATH)
    if (gflags_FOUND)
        message(STATUS "Found installed version of gflags: ${gflags_DIR}")
    else(gflags_FOUND)
        # Failed to find an installed version of gflags, repeat search allowing
        # exported build directories.
        message(STATUS "Failed to find installed gflags CMake configuration, "
                "searching for gflags build directories exported with CMake.")
        # Again pass NO_CMAKE_BUILDS_PATH, as we know that gflags is exported and
        # do not want to treat projects built with the CMake GUI preferentially.
        find_package(gflags QUIET
                NO_MODULE
                NO_CMAKE_BUILDS_PATH)
        if (gflags_FOUND)
            message(STATUS "Found exported gflags build directory: ${gflags_DIR}")
        endif(gflags_FOUND)
    endif(gflags_FOUND)
    set(FOUND_INSTALLED_GFLAGS_CMAKE_CONFIGURATION ${gflags_FOUND})
    # gflags v2.1 - 2.1.2 shipped with a bug in their gflags-config.cmake [1]
    # whereby gflags_LIBRARIES = "gflags", but there was no imported target
    # called "gflags", they were called: gflags[_nothreads]-[static/shared].
    # As this causes linker errors when gflags is not installed in a location
    # on the current library paths, detect if this problem is present and
    # fix it.
    #
    # [1] https://github.com/gflags/gflags/issues/110
    if (gflags_FOUND)
        # NOTE: This is not written as additional conditions in the outer
        #       if (gflags_FOUND) as the NOT TARGET "${gflags_LIBRARIES}"
        #       condition causes problems if gflags is not found.
        if (${gflags_VERSION} VERSION_LESS 2.1.3 AND
                NOT TARGET "${gflags_LIBRARIES}")
            message(STATUS "Detected broken gflags install in: ${gflags_DIR}, "
                    "version: ${gflags_VERSION} <= 2.1.2 which defines gflags_LIBRARIES = "
                    "${gflags_LIBRARIES} which is not an imported CMake target, see: "
                    "https://github.com/gflags/gflags/issues/110.  Attempting to fix by "
                    "detecting correct gflags target.")
            # Ordering here expresses preference for detection, specifically we do not
            # want to use the _nothreads variants if the full library is available.
            list(APPEND CHECK_GFLAGS_IMPORTED_TARGET_NAMES
                    gflags-shared gflags-static
                    gflags_nothreads-shared gflags_nothreads-static)
            foreach(CHECK_GFLAGS_TARGET ${CHECK_GFLAGS_IMPORTED_TARGET_NAMES})
                if (TARGET ${CHECK_GFLAGS_TARGET})
                    message(STATUS "Found valid gflags target: ${CHECK_GFLAGS_TARGET}, "
                            "updating gflags_LIBRARIES.")
                    set(gflags_LIBRARIES ${CHECK_GFLAGS_TARGET})
                    break()
                endif()
            endforeach()
            if (NOT TARGET ${gflags_LIBRARIES})
                message(STATUS "Failed to fix detected broken gflags install in: "
                        "${gflags_DIR}, version: ${gflags_VERSION} <= 2.1.2, none of the "
                        "imported targets for gflags: ${CHECK_GFLAGS_IMPORTED_TARGET_NAMES} "
                        "are defined.  Will continue with a manual search for gflags "
                        "components.  We recommend you build/install a version of gflags > "
                        "2.1.2 (or master).")
                set(FOUND_INSTALLED_GFLAGS_CMAKE_CONFIGURATION FALSE)
            endif()
        endif()
    endif()
    if (FOUND_INSTALLED_GFLAGS_CMAKE_CONFIGURATION)
        message(STATUS "Detected gflags version: ${gflags_VERSION}")
        set(GFLAGS_FOUND ${gflags_FOUND})
        set(GFLAGS_INCLUDE_DIR ${gflags_INCLUDE_DIR})
        set(GFLAGS_LIBRARY ${gflags_LIBRARIES})
        # gflags does not export the namespace in their CMake configuration, so
        # use our function to determine what it should be, as it can be either
        # gflags or google dependent upon version & configuration.
        #
        # NOTE: We use the regex method to determine the namespace here, as
        #       check_cxx_source_compiles() will not use imported targets, which
        #       is what gflags will be in this case.
        gflags_check_gflags_namespace_using_regex()
        if (NOT GFLAGS_NAMESPACE)
            gflags_report_not_found(
                    "Failed to determine gflags namespace using regex for gflags "
                    "version: ${gflags_VERSION} exported here: ${gflags_DIR} using CMake.")
        endif (NOT GFLAGS_NAMESPACE)
    else (FOUND_INSTALLED_GFLAGS_CMAKE_CONFIGURATION)
        message(STATUS "Failed to find an installed/exported CMake configuration "
                "for gflags, will perform search for installed gflags components.")
    endif (FOUND_INSTALLED_GFLAGS_CMAKE_CONFIGURATION)
endif(GFLAGS_PREFER_EXPORTED_GFLAGS_CMAKE_CONFIGURATION)
if (NOT GFLAGS_FOUND)
    # Either failed to find an exported gflags CMake configuration, or user
    # told us not to use one.  Perform a manual search for all gflags components.
    # Handle possible presence of lib prefix for libraries on MSVC, see
    # also GFLAGS_RESET_FIND_LIBRARY_PREFIX().
    if (MSVC)
        # Preserve the caller's original values for CMAKE_FIND_LIBRARY_PREFIXES
        # s/t we can set it back before returning.
        set(CALLERS_CMAKE_FIND_LIBRARY_PREFIXES "${CMAKE_FIND_LIBRARY_PREFIXES}")
        # The empty string in this list is important, it represents the case when
        # the libraries have no prefix (shared libraries / DLLs).
        set(CMAKE_FIND_LIBRARY_PREFIXES "lib" "" "${CMAKE_FIND_LIBRARY_PREFIXES}")
    endif (MSVC)
    # Search user-installed locations first, so that we prefer user installs
    # to system installs where both exist.
    list(APPEND GFLAGS_CHECK_INCLUDE_DIRS
            /usr/local/include
            /usr/local/homebrew/include # Mac OS X
            /opt/local/var/macports/software # Mac OS X.
            /opt/local/include
            /usr/include)
    list(APPEND GFLAGS_CHECK_PATH_SUFFIXES
            gflags/include # Windows (for C:/Program Files prefix).
            gflags/Include ) # Windows (for C:/Program Files prefix).
    list(APPEND GFLAGS_CHECK_LIBRARY_DIRS
            /usr/local/lib
            /usr/local/homebrew/lib # Mac OS X.
            /opt/local/lib
            /usr/lib)
    list(APPEND GFLAGS_CHECK_LIBRARY_SUFFIXES
            gflags/lib # Windows (for C:/Program Files prefix).
            gflags/Lib ) # Windows (for C:/Program Files prefix).
    # Search supplied hint directories first if supplied.
    find_path(GFLAGS_INCLUDE_DIR
            NAMES gflags/gflags.h
            PATHS ${GFLAGS_INCLUDE_DIR_HINTS}
            ${GFLAGS_CHECK_INCLUDE_DIRS}
            PATH_SUFFIXES ${GFLAGS_CHECK_PATH_SUFFIXES})
    if (NOT GFLAGS_INCLUDE_DIR OR
            NOT EXISTS ${GFLAGS_INCLUDE_DIR})
        gflags_report_not_found(
                "Could not find gflags include directory, set GFLAGS_INCLUDE_DIR "
                "to directory containing gflags/gflags.h")
    endif (NOT GFLAGS_INCLUDE_DIR OR
            NOT EXISTS ${GFLAGS_INCLUDE_DIR})
    find_library(GFLAGS_LIBRARY NAMES gflags
            PATHS ${GFLAGS_LIBRARY_DIR_HINTS}
            ${GFLAGS_CHECK_LIBRARY_DIRS}
            PATH_SUFFIXES ${GFLAGS_CHECK_LIBRARY_SUFFIXES})
    if (NOT GFLAGS_LIBRARY OR
            NOT EXISTS ${GFLAGS_LIBRARY})
        gflags_report_not_found(
                "Could not find gflags library, set GFLAGS_LIBRARY "
                "to full path to libgflags.")
    endif (NOT GFLAGS_LIBRARY OR
            NOT EXISTS ${GFLAGS_LIBRARY})
    # gflags typically requires a threading library (which is OS dependent), note
    # that this defines the CMAKE_THREAD_LIBS_INIT variable.  If we are able to
    # detect threads, we assume that gflags requires it.
    find_package(Threads QUIET)
    set(GFLAGS_LINK_LIBRARIES ${CMAKE_THREAD_LIBS_INIT})
    # On Windows (including MinGW), the Shlwapi library is used by gflags if
    # available.
    if (WIN32)
        include(CheckIncludeFileCXX)
        check_include_file_cxx("shlwapi.h" HAVE_SHLWAPI)
        if (HAVE_SHLWAPI)
            list(APPEND GFLAGS_LINK_LIBRARIES shlwapi.lib)
        endif(HAVE_SHLWAPI)
    endif (WIN32)
    # Mark internally as found, then verify. GFLAGS_REPORT_NOT_FOUND() unsets
    # if called.
    set(GFLAGS_FOUND TRUE)
    # Identify what namespace gflags was built with.
    if (GFLAGS_INCLUDE_DIR AND NOT GFLAGS_NAMESPACE)
        # To handle Windows peculiarities / CMake bugs on MSVC we try two approaches
        # to detect the gflags namespace:
        #
        # 1) Try to use check_cxx_source_compiles() to compile a trivial program
        #    with the two choices for the gflags namespace.
        #
        # 2) [In the event 1) fails] Use regex on the gflags headers to try to
        #    determine the gflags namespace.  Whilst this is less robust than 1),
        #    it does avoid any interaction with msbuild.
        gflags_check_gflags_namespace_using_try_compile()
        if (NOT GFLAGS_NAMESPACE)
            # Failed to determine gflags namespace using check_cxx_source_compiles()
            # method, try and obtain it using regex on the gflags headers instead.
            message(STATUS "Failed to find gflags namespace using using "
                    "check_cxx_source_compiles(), trying namespace regex instead, "
                    "this is expected on Windows.")
            gflags_check_gflags_namespace_using_regex()
            if (NOT GFLAGS_NAMESPACE)
                gflags_report_not_found(
                        "Failed to determine gflags namespace either by "
                        "check_cxx_source_compiles(), or namespace regex.")
            endif (NOT GFLAGS_NAMESPACE)
        endif (NOT GFLAGS_NAMESPACE)
    endif (GFLAGS_INCLUDE_DIR AND NOT GFLAGS_NAMESPACE)
    # Make the GFLAGS_NAMESPACE a cache variable s/t the user can view it, and could
    # overwrite it in the CMake GUI.
    set(GFLAGS_NAMESPACE "${GFLAGS_NAMESPACE}" CACHE STRING
            "gflags namespace (google or gflags)" FORCE)
    # gflags does not seem to provide any record of the version in its
    # source tree, thus cannot extract version.
    # Catch case when caller has set GFLAGS_NAMESPACE in the cache / GUI
    # with an invalid value.
    if (GFLAGS_NAMESPACE AND
            NOT GFLAGS_NAMESPACE STREQUAL "google" AND
            NOT GFLAGS_NAMESPACE STREQUAL "gflags")
        gflags_report_not_found(
                "Caller defined GFLAGS_NAMESPACE:"
                " ${GFLAGS_NAMESPACE} is not valid, not google or gflags.")
    endif ()
    # Catch case when caller has set GFLAGS_INCLUDE_DIR in the cache / GUI and
    # thus FIND_[PATH/LIBRARY] are not called, but specified locations are
    # invalid, otherwise we would report the library as found.
    if (GFLAGS_INCLUDE_DIR AND
            NOT EXISTS ${GFLAGS_INCLUDE_DIR}/gflags/gflags.h)
        gflags_report_not_found(
                "Caller defined GFLAGS_INCLUDE_DIR:"
                " ${GFLAGS_INCLUDE_DIR} does not contain gflags/gflags.h header.")
    endif (GFLAGS_INCLUDE_DIR AND
            NOT EXISTS ${GFLAGS_INCLUDE_DIR}/gflags/gflags.h)
    # TODO: This regex for gflags library is pretty primitive, we use lowercase
    #       for comparison to handle Windows using CamelCase library names, could
    #       this check be better?
    string(TOLOWER "${GFLAGS_LIBRARY}" LOWERCASE_GFLAGS_LIBRARY)
    if (GFLAGS_LIBRARY AND
            NOT "${LOWERCASE_GFLAGS_LIBRARY}" MATCHES ".*gflags[^/]*")
        gflags_report_not_found(
                "Caller defined GFLAGS_LIBRARY: "
                "${GFLAGS_LIBRARY} does not match gflags.")
    endif (GFLAGS_LIBRARY AND
            NOT "${LOWERCASE_GFLAGS_LIBRARY}" MATCHES ".*gflags[^/]*")
    gflags_reset_find_library_prefix()
endif(NOT GFLAGS_FOUND)
# Set standard CMake FindPackage variables if found.
if (GFLAGS_FOUND)
    set(GFLAGS_INCLUDE_DIRS ${GFLAGS_INCLUDE_DIR})
    set(GFLAGS_LIBRARIES ${GFLAGS_LIBRARY} ${GFLAGS_LINK_LIBRARIES})
endif (GFLAGS_FOUND)
# Handle REQUIRED / QUIET optional arguments.
include(FindPackageHandleStandardArgs)
find_package_handle_standard_args(gflags DEFAULT_MSG
        GFLAGS_INCLUDE_DIRS GFLAGS_LIBRARIES GFLAGS_NAMESPACE)
# Only mark internal variables as advanced if we found gflags, otherwise
# leave them visible in the standard GUI for the user to set manually.
if (GFLAGS_FOUND)
    mark_as_advanced(FORCE GFLAGS_INCLUDE_DIR
            GFLAGS_LIBRARY
            GFLAGS_NAMESPACE
            gflags_DIR) # Autogenerated by find_package(gflags)
endif (GFLAGS_FOUND)
